There are numerous applications in which it is desirable to detect small changes, either electrical or optical, in the operating parameters of semiconductor lasers. Measurements of these parameters, as well as perhaps other parameters, are typically carried out with the laser operating at both fixed temperature and output power using direct detection of the laser output. Fixed power is typically maintained with automatic power control (APC).
Fixed temperature and power operation is also used with the light source in an optical time domain reflectometer which determines the distance from the light source to, for example, a reflecting surface in an optical fiber by measuring the elapsed time between pulse emission and return. In this application, changes in the operating parameters are not measured.
An important application of operating parameter measurements is the determination of semiconductor laser reliability or lifetime. While lifetime measurements are important for virtually all laser applications, they are especially important for optical communications applications, such as submarine cables, where laser replacement is both difficult and expensive.
Lifetime measurements typically rely on a technique, called accelerated aging, which operates a semiconductor laser for a prolonged period of time at a temperature elevated with respect to the normal operating temperature. Changes in the electrical input power of the laser are detected as constant optical output power is maintained. While perfectly adequate for many purposes, this technique suffers from the drawback that the degradation mechanism operative at the elevated temperature need not be the degradation mechanism operative at the intended use temperature. There is, therefore, an element of uncertainty in the lifetime results obtained.
The measurement of semiconductor laser parameters is made still more difficult by the discovery that nonlinear optical cavity effects produce instabilities in the laser APC system. Thus, the accuracy of the parameter measurements is limited by the control system's stability. The control system has equilibrium points which are the solutions to the closed loop control system. An equilibrium point is considered stable if the system ultimately approaches that point. The concept of stability thus means, in practical terms, the behavior of the control system in the neighborhood of an equilibrium point. However, if the entire system has a single nonlinear element, there may be regions in which there is no equilibrium point. It should also be noted that some lasers may fluctuate between multiple equilibrium points and reduce the signal to noise ratio. For example, there may be discontinuities in the light output versus current characteristic curve. This condition is made worse when, as inevitably happens, a small perturbation disturbs the system equilibrium and causes a temporal discontinuity in the parameter measurement. Thus, the precision of the measurements is also limited by one's ability to distinguish between instabilities in the device under test and the control system. Uncertainties in the parameter measurements and the projected lifetimes inevitably result.